LONDON, Aug. 10 — British authorities said today that they had thwarted a terrorist plot to use liquid explosives concealed in carry-on luggage to blow up airliners traveling between Britain and the United States. A police official in London called it a plan for “mass murder on an unimaginable scale.” In a briefing at Scotland Yard, the police said they had arrested 21 people in connection with the plot. In response, some flights into London’s Heathrow Airport were canceled. Airlines banned all carry-on luggage on planes leaving Heathrow, causing chaos and long delays during one of the busiest weeks for summer trans-Atlantic travel. In the United States, federal officials raised the security warning level for all aviation to high, and to severe for flights arriving from the United Kingdom. They also put in place new regulations barring passengers from carrying any liquids, gels or lotions onto planes, except for milk or juice for young children and medicines. Homeland Security Secretary Michael Chertoff said the restrictions reflected the belief of investigators that the plotters planned to bring liquids on board, “each one of which would be benign, but mixed together could be used to create a bomb.’’ The liquids were to be disguised as beverages and the detonators as “electronic devices or other common devices.’’ Britain banned all cell phones and portable music players from flights. Mr. Chertoff called the plot “a very sophisticated plan and operation’’ that was close to fruition. “They had accumulated the capability necessary and they were well on their way,’’ he said at a televised news conference in Washington. In London, Peter Clarke, the head of the counterterrorism branch of the metropolitan police, suggested that the group had not yet constructed its bombs. “The intelligence suggested that the devices were to be constructed in the United Kingdom and taken through British airports,’’ he told reporters. Aviation experts have long known that planes could be vulnerable to explosive devices put together on board or from hazardous liquids, said Robert W. Mann Jr., an industry consultant based in Port Washington, N.Y. “Yes, it could have happened,’’ he said. Mr. Chertoff said that the plot appeared to have been aimed at U.S. carriers flying out of Heathrow. He did not name specific airlines, but the principal ones serving those routes are American Airlines, Continental Airlines and United Airlines. The director of the Federal Bureau of Investigation, Robert S. Mueller III, said that the plot “had all the earmarks of an al Qaeda plot,’’ but he said there was no direct evidence linking the suspects to the terrorist organization. In 1994, al Qaeda members in the Philippines planned to blow up airliners in simultaneous attacks over the Pacific, a plot that was disrupted by the arrest of its ringleader. In London today, the police did not identify the suspects or their origin, though Paul Stephenson, the deputy metropolitan police commissioner, said “community leaders” had been alerted about the police action, using a code word for the British Muslim community. A police official told Reuters that some of the suspects were British. Authorities in London did not say how many aircraft had been identified for attack. Sky News put the number at six, while other reports said between three and 10. Mr. Chertoff spoke of “multiple explosions in multiple planes.’’ Mr. Stephenson spoke of “an extraordinarily serious plot.’’ “We are confident that we have stopped an attempt to create mass murder on an unimaginable scale,” he said. He said the people had been arrested in and near London and Birmingham, and added that the searches would continue. “We have been very successful in arresting those we were targeting, but this is a lengthy operation, and no doubt there will be further developments,” he told reporters. Britain’s Home Secretary, John Reid, said that “we think that the main players are already in custody.” “But we should always err on the side of caution,” he added, explaining why the threat level remains elevated. Mr. Stephenson said that “a critical point” was reached last night that indicated “an urgent need to take action,” but he declined to say what information led to that decision. Mr. Chertoff said that “some threads had been pursued for some time’’ by investigators in Britain, but that in the last two weeks or less evidence had arisen suggesting that the plotters were targeting American carriers. He said there was no evidence of any related plots in the United States, or any threats against other modes of transportation. Mr. Clarke told reporters that the investigation had “already lasted for several months and will undoubtedly last long into the future.’’ He said the investigation had involved “an unprecedented level of surveillance.’’ “We have been looking at meetings, movement, travel, spending and the aspirations of a large group of people,’’ Mr. Clarke said. The British Broadcasting Service reported that the police were evacuating homes in High Wycombe, about 30 miles from London, near one of the houses being searched.